Surgical drainage bags such as are commonly used by ileostomy or colostomy patients are often known as "ostomy" bags and that term will be used herein.
Ostomy bags commonly comprise two sheets of a flexible plastics material arranged in face contact and welded or otherwise secured at the edges. This produces a bag with two walls which are in face contact until the bag is filled. One of these walls, herein considered to be the rear wall, contacts the body of the patient when the bag is in use. The rear wall of such a bag has an inlet capable of receiving the stoma of a patient and the contents of the intestine of the patient can pass through this inlet into the interior of the bag. The contents flowing into the bag include flatus which would undesirably inflate the bag if it was not allowed to escape and for this purpose many such bags are provided with a vent through which these gases may pass. It is well known for this vent to include a deodourising filter which may, if desired, be constructed as described in our German Offenlegungsschrift Pulbication No. 2730286.9. However, the ability of such filters to pass gases freely depends on the pore size of the component parts. Thus, a filter that is sufficiently porous to allow a satisfactory flow of gas may well not prevent the transmission of liquid through the filter with the result that the filter tends to leak. The present invention seeks to overcome this disadvantage.